Copper etching is a complex oxidation-reduction process in which copper is converted from the metallic state into the ionic state, and the oxidizing agent is reduced. Acid solutions containing copper chloride are extensively used as etching solutions in the copper etching process. One such acid solution commonly used to etch printed circuit boards is a cupric chloride-based hydrochloric acid solution comprising water, HCl, CuCl2 and oxidizers.
The usual etching procedure involves placing a resist pattern over a sheet of laminated copper. The masked copper laminate is then brought into contact with the etching solution (i.e., the etchant), which dissolves the exposed copper and leaves behind the copper which is protected by the resist pattern. The etchant comprises hydrochloric acid and an oxidizer, and also typically cleans brighteners, inhibitors and the like. The reactions of the etching process that generate the waste cupric chloride are believed to be:[CuCl2+2HCl (aqueous)]+Cu0→2HCuCl2(aqueous)  Reaction 12HCuCl2(aqueous)+(oxidizing agent)→2CuCl2(aqueous)  Reaction 2
The copper etching process generally operates in the continuous mode. Hydrochloric acid and an oxidizer are added continuously, while a waste stream comprising cupric chloride is bled off continuously. Oxidizers used in the etching process can include, but are not limited to, hydrogen peroxide, chlorine, or sodium chlorate. The waste copper chloride generated in the etching process is typically shipped off-site for reclamation of the copper. This waste copper chloride solution is generally treated as a hazardous waste under the Resource Conservation and Recovery Act (RCRA), which increases the cost of shipping and handling. Additionally, waste chloride ions typically exit the process as a waste brine. Due to the high costs of shipping, handling, and treating these waste streams, there is more interest in on-site recycling.
Various processes have been used to treat other acidic waste streams. “Treatment of Acid Pickling Waste of Metals” by Bandyopadhyay in the Journal of Indian Pollution Control, 15(2), pages 259–265, 1999, discloses treatment processes for acid-based cleaning operations used in metal industries. Such treatment processes include neutralization, sedimentation, crystallization, and acid recovery. U.S. Pat. No. 3,635,664 to Morimoto discloses a process to regenerate a hydrochloric acid pickling waste stream. The process is used to convert FeCl2 in the waste stream to FeSO4. Similarly, “Recovery of Acid from Pickling Liquors” by Kumar et al. in Envron. Eng. Sci. 15(4), pages 259–260, 1998 discloses a resource recovery process to treat HCl-spent pickle liquors to recover 85% of the HCl and 86.5% pure FeSO4.
U.S. Pat. No. 5,013,395 to Blumberg et al. discloses a method of regenerating a metal-containing acid solution enhanced with a salt-free material. The metal dissolved in the acid solution is continuously oxidized by introducing a gas into a packed reaction vessel. U.S. Pat. No. 5,500,098 to Brown et al. discloses a process and apparatus for regenerating volatile acids containing metal salt impurities. U.S. Pat. No. 5,560,838 to Allies et al. discloses a process for converting an etchant into a non-hazardous material through a reaction with heated caustic (sodium hydroxide).
Generally, however, processes relating to copper have been more complex. U.S. Pat. No. 4,604,175 to Naumov et al. discloses an electrochemical reduction and oxidation (redox) process for regenerating an iron-copper chloride etching solution. U.S. Pat. No. 5,421,966 to Oxley discloses an electrolytic apparatus and process for the on-line regeneration of acidic cupric chloride etching baths. A preferred system utilizes a flow-through graphite or carbon anode and a flow-by cathode to allow more precise control of current and voltage. JP 04089316 to Sakata et al. discloses a method of recovering copper salts from hydrchloric acid-containing solutions, wherein solutions are dehydrochlorinated by electrodialysis or diffusion dialysis to remove the HCL. JP 11158661 to Hosoda discloses a process of extracting copper from a spent etching solution using acidic-based extractant solutions and electrolyzing the copper-containing acid solution to recover the copper.
The aforementioned references have inherent problems. For example, the energy costs associated with the electrochemical or electroytic processes for regenerating acidic etchant solutions are prohibitively high. The energy and handling costs associated with additional steps, such as evaporation, in distillation-based processes are also high. The shipping, handling, and transportation costs associated with caustic-based processes are also very high. Thus, there remains a need for an improved process to reclaim the materials in cupric chloride-based acid solutions, especially for a cost efficient process to reclaim usable or marketable products such as copper sulfate and hydrochloric acid from spent chloride-based copper etchant solutions.